Chemical indicators are generally used to monitor whether or not an article, such as medical apparatus, has been exposed to sterilizing conditions. The type of chemical indicator discussed here is commonly known as a "throughput indicator." This type of indicator responds to one or more physical or chemical components in the sterilization environment, but it does not necessarily have to respond to all the components necessary for sterilization. Since the indicator response may occur in the absence of one or more essential sterilization components, the indicator response is not necessarily an indication of sterility. The chemical indicator only indicates that it and the accompanying articles have been processed in a sterilizer.
Often, the indicator takes the form of a reactive chemical which reacts with a chemical in the sterilizing environment to form a new chemical with different properties, frequently a different color. Other indicators react to physical conditions encountered during sterilization; for instance, wax indicators melt when exposed to the high temperatures encountered during autoclaving. Descriptions of various chemical indicators may be found in the monograph "Biological and Chemical Indicators," Report 78-4.4 of the Health Industry Manufacturer's Association (HIMA).
Among the important performance characteristics for chemical indicators are readability, reliability, selectivity, stability, and safety. "Readability" refers to the indicator characteristics which allow users to differentiate between unexposed indicators and those which have been exposed to sterilization conditions. "Good readability" means that virtually all healthcare workers, including those with common vision problems such as color blindness, can distinguish exposed indicators from unexposed indicators. For example, readability may be difficult when a user attempts to determine the shade of a color such as light brown turning to medium brown as the color change. Further, indicators should change in contrast as well as color. An indicator which changed, for example, from red to green, could pose problems for users with color vision defects unless there was an accompanying (sufficient) contrast change.
Stability is also an important characteristic. The indicating means of both unexposed and exposed indicators must not change under the normal storage conditions the article encounters. One type of commercially available chemical indicator, for example, must be stored in a refrigerator to prevent incorrect color changes over normal storage, which is inconvenient and disadvantageous.
Chemical indicators have been developed and are used with both of the prevalent sterilization processes: steam and ethylene oxide. For example, U.S. Pat. No. 4,914,034, issued Apr. 3, 1990, inventors Welsh and Dyke, describes disposable test packs for monitoring steam and ethylene oxide sterilization cycles, which include a chemical process indicator strip. U.S. Pat. No. 4,671,936, issued Jun. 9, 1987, inventor Barron, describes a cation exchange resin for monitoring alkylene oxide cycles (e.g. ethylene oxide).
Recently, a process for sterilization that includes exposing an article in a sterilizing chamber under reduced pressure to at least one exposure cycle of a peracid antimicrobial agent has been described by U.S. Pat. No. 5,084,239, issued Jan. 28, 1992, inventors Moulton et al. This sterilization process holds promise as replacement for the presently used ethylene oxide sterilization processes, which are being subjected to increasing scrutiny and safety concerns because ethylene oxide is a carcinogen, and as an alternative to conventional steam sterilization also. Although steam sterilization processes are effective for medical devices such as metal implements, the elevated temperatures to which articles are exposed during steam processing do not permit sterilization of many synthetic (especially plastic) materials or devices packaged in temperature sensitive wraps.
Accordingly, there is a need for suitable chemical indicators for monitoring exposure to a sterilization process where the sterilization cycle includes an acidic gas or vapor, particularly when the cycle is conducted under reduced pressure.